1. Field of Invention
This invention relates to fluid additive supply systems for fire fighting equipment, and in particular to systems for adding foam concentrate into a water stream used for fire fighting operations.
2. Description of Related Art
Fire fighting mechanisms typically include a source of water (the primary fire fighting fluid) connected to a water pump that supplies water via a conduit (fire hose) to a discharge device (hand held nozzle, monitor, sprinkler system, foam chambers, etc.).
It is often desirable to have a mechanism that can supply an additive, such as foam concentrate, into the water stream. Foam is useful in extinguishing certain types or classes of fires, particularly flammable liquid fires. The additive supply mechanisms may have a metering device that allows the introduction certain percentages of foam to water, termed proportioning, to be varied (i.e. 1%, 3%, 6% foam to water proportioning are the most common).
There are numerous systems on the market that supply additives into water streams. The majority of these systems are in fixed installations or permanently mounted in fire fighting apparatus such as a fire truck. Such apparatus for foam proportioning are discussed and disclosed, for instance, in U.S. Pat. No. 4,436,487. Many of these systems require the use of foam concentrate additive pumps for forced injection. These pumps may be, for example, either positive displacement or centrifugal type. Such systems designed for foam proportioning are complicated and very expensive.
Alternatively, the basic design and principle of a venturi device can be used to "induct" or "educt" foam concentrate into the primary water stream Venturi mechanisms use primary liquid flow to create a pressure drop across an orifice. The pressure drop forms a partial vacuum into which fluid or gases will flow if permitted. Additives supplied in the vicinity of the venturi mechanism will be pulled into the partial vacuum created.
A close relative of the present invention is termed an "around the pump" foam system. These systems involve the use of jet pumps and/or eductors together with a pump. The "around the pump" foam system is setup as follows (FIG. 1). The fire fighting apparatus connects a pressurized water source such as from a fire hydrant 1 to the suction inlet side 6 of the water pump 2 via a water conduit line 7. A further water conduit line 8 attaches to the discharge outlet side of the water pump 2 and connects to a deployed downstream fire fighting fluid delivery device such as for example a hand held nozzle 3, fixed nozzle termed a "monitor", or a sprinkler system. An auxiliary conduit 10 from the discharge outlet side 9 of the water pump is connected to a jet pump and/or eductor 4. The discharge conduit 11 from the jet pump and/or eductor 4 is attached to an auxiliary connection on the suction inlet side of the water pump 12. Once water flow is established, the auxiliary discharge conduit 10 supplies water to the inlet of the jet pump and/or eductor 4. The water passes through a pressure drop area. The additive 5 is drawn from a suction port inlet 13 that is connected to the device in the low pressure zone that is created by this pressure drop. The additive is entrained by the operating water stream of the jet pump and/or eductor. The entrained foam concentrate mixing together with the jet pump and/or eductor operating water stream is then injected into the auxiliary connection 12 on the suction side of the pump 2. The total solution than passes through the water pump where a pressure increase is effected. Once adequately pressurized, the water/additive solution is then directed into the deployed downstream fire fighting fluid delivery device 3. The "around the pump" foam system can either be installed as a fixed installation on the water pump apparatus or can be deployed portably.
Some inherent problems associated with the "around the pump" foam system are as follows. The ability for the system to operate is based on the performance of the jet pump/eductor. The jet pump/eductor must have the energy (discharge head pressure) to inject the entrained foam into the pressurized system of the water pump. The available energy is based on the style of the jet pump/eductor and the operating water pressure supplied to the inlet of the jet pump/eductor, the amount of additive picked up, and the discharge velocity of the homogenous fluid mixture(i.e. operating water and additive). As a rule jet pumps inducting a high relative percent of additive fluid have discharge capabilities of approximately 30% of inlet pressure. Full in-line eductors inducing a low relative percent of additive have discharge capabilities of approximately 65% of inlet pressure. In the relevant fire fighting systems of the present invention, additive pick up is normally approximately 1.8-15 gpm (gallons per minute) (based on different eductor models with flow capabilities ranging from 30-250 gpm at 6% proportioning).
Once the pressure on the inlet side of the water pump exceeds the discharge head pressure, the proportioning of foam will either decrease or totally cease, usually the latter. Thus, if the supply water conduit to the pump system inlet has an inlet pressure exceeding the discharge capabilities of the jet pump/eductor, as is often the case, a manual control valve will need to be adjusted, such valve is installed between the water supply and the inlet side of the water pump. The port through the valve will be reduced to adjust the water pressure at the pump inlet so as to not exceed the discharge head capabilities of the jet pump/eductor.
Inlet water pump pressure variation is usually caused by the increase or decrease in total volume through the water pump. Once the manual control valve is adjusted for the determined flow, any deviation of flow will cause a variation in pressure. Many times this variation is enough to exceed the capabilities of the jet pump/eductor and causes system failure. Volume variations are normally due to the opening and closing of the downstream water stream delivery devices. Thus, the water pump operator must constantly monitor the discharge water volume and adjust the manual control valve accordingly. Without this constant adjustment the inlet water pressure to the pump will either be in excess of the jet pump system, or will not have adequate volume to supply the discharge requirements, resulting in cavitation.
Another deficiency in the current "around the pump" foam system relates to fluctuation in the percentage of additive concentrate. The fluctuation is due to changing water volumes created by the opening and closing of the downstream fluid delivery devices. Once the desired amount of concentrate is selected, the volume of concentrate added is fixed at the optimum proportioning level. If the volume of water passing through the water pump changes and the amount of additive remains the same than the percent (it additive will vary immensely. An example of this is as follows.